The invention relates generally to an electronic camera and more particularly to an electronic camera employing a charge coupled device (CCD) for imaging and recording an image. Conventional digital video cameras generate a video signal by employing such a CCD image pickup device to store a plurality of image pixels, each corresponding to one of a plurality of image elements within the CCD device. Typically, each of these imaging elements comprises a MOS capacitor arranged in a grid. Each MOS capacitor performs a photoelectric conversion, a charge accumulation, and a charge transfer. When light shines on a particular MOS capacitor, the capacitor creates a signal charge which varies directly with the amount of incident light. The amount of charge that is able to be stored in a particular MOS capacitor can be defined by the application of a predetermined voltage to the MOS capacitor. Thus, by the application of a predetermined voltage a predetermined amount of charge can be stored in this MOS capacitor. Such a charge holder is called a xe2x80x9cwellxe2x80x9d. Once a charge has been stored within the capacitor, it is possible to transfer this charge to another capacitor well, if the other well is deeper (i.e. has a greater voltage applied thereto). This characteristic allows for the systematic storage of a charge related to an amount of light received at each of a plurality of CCD imaging elements, and thereafter the transfer of the stored charges from each imaging element through an appropriate system to record the amount of light received by each capacitor in an imaging system to reproduce the image pixel by pixel.
Each capacitor unit of the CCD imaging device is designed to receive an amount of light within a predetermined range. Thus, the well formed by each capacitor unit receives light, moving from an empty charge state to a full charge state. After a particular capacitor unit receives enough light that its well is full, additional charge is siphoned off and dumped to an overflow drain. Thus, a maximum storable charge has been exceeded. Upon overflow, the display of a pixel will all appear plain white. A problem arises in that if all of the capacitors in an area of an image have received more than a predetermined amount of light, thus exceeding the maximum charge, all of these pixels will appear white, with no distinction between them. This is because once an element has received all of the light it is designed to receive, a charge completely filling its well is generated. The pixel is thus 100% white, and no additional light can be received thereby or charge can be stored therein. Thus, where there may be an actual difference between the light received by various pixels of the imaging device, once reaching a maximum, the CCD device determines that each of these elements should be displayed identically. Thus, differences between light received by pixels that is above an upper threshold is not accounted for.
In order to remedy this situation, conventional CCD cameras allow for the use of a shutter, either physical or electronic in order to reduce the light received by the entire CCD device. When using a physical shutter, the amount of light allowed to enter the lens is reduced, by reducing the aperture size of the camera as in a typical chemical photography camera. Thus, the size of the objective lens is stopped down, and therefore less light is received. Alternatively, it is possible to employ an electronic shutter, in which a reduced amount of time is allowed for light to reach each of the imaging elements. In this manner, overflow is avoided.
However, in each of these cases, either the physical or electronic shutter acts on the entire image. Therefore, if certain portions of the image are bright, while others are dark, the use of the shutter to reduce the amount of light received by the entire imaging device, while possibly improving the resolution between pixels at the bright end of the spectrum, is likely to reduce the resolution of images at the darker end of the spectrum. If a user is taking a picture of an image that includes bright sunlight, and a shadow, typically the user will have to choose between being able to distinguish elements within the shadow or elements within the bright light.
Therefore, it would be beneficial to provide an improved method and apparatus that overcomes these drawbacks of the prior art.
It is an object of the invention to provide an improved CCD imaging device capable of accurately recording an image including a variety of light conditions.
Another object of the invention is to provide an improved CCD imaging device in which a portion of an image having a high light content, and a portion of an image having a low light content can each be imaged and stored simultaneously at an acceptable resolution and contrast.
A further object of the invention is to provide an improved CCD imaging device that determines portions of a picture to be imaged that are candidates for reduction in the amount of light to be received, and other portions of the image that are candidates for increases in the amount of light received, and independently adjusts the CCD elements corresponding to these portions of the picture accordingly.
A still further object of the invention is to provide an improved CCD imaging device which allows for the automatic detection and control of various areas of a picture to be imaged, and which adjusts the amount of time available in each predetermined area for receiving light from the picture to be imaged in order to improve the resolution between pixels within each area.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification and the drawings.
Generally speaking, in accordance with the invention, an improved CCD imaging device is provided for imaging and recording a received picture. The improved CCD imaging device in accordance with the invention is constructed so that first a picture to be stored is recorded and tested. During this test, areas of the image that may produce an overflow of the CCD imaging pixel elements, or which may not produce enough light for the CCD imaging pixel elements are identified. An edge detector is then utilized to determine the boundaries between these particular areas and the remainder of the picture. After determining various areas which are to have the amount of light received thereby altered, a determination is made as to what percentage of the received light should be received upon correction. Thus, very bright subjects will be corrected to receive a smaller amount of the normally received light, while very dark areas in the picture will be adjusted to receive more than a normal amount of light. After correction of the various areas, subsequent images are stored, and thus areas which would typically be too bright are reduced in light to provide proper contrast, while areas that would typically be too dark are also adjusted to provide proper contrast. In this manner a selective electronic shutter is applied to various portions of a picture to be imaged and recorded, thereby allowing for changes in exposure time (or other mechanisms) for adjusting the amount of light received by a particular pixel without adjusting the amount of light received by other pixels in the image.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangement of parts that are adopted to affect such steps, all as exemplified in the followed detailed disclosure, and the scope of the invention will be indicated in the claims.